It is known to use an oxidizing bleaching agent, primarily some peroxide, and a reducing bleaching agent, primarily dithionite, in the manufacture of bleached mechanical pulp, for instance thermomechanical pulp. It is also known to bleach one and the same mechanical pulp with both types of bleaching agent, i.e. in an oxidizing bleaching stage followed by a reducing bleaching stage, or vice versa.
This also applies to the manufacture of chemithermomechanical pulp. Peroxide, normally hydrogen peroxide, is a highly effective bleaching agent that bleaches the pulp to high brightness. However, peroxide bleaching normally requires the use of separate bleaching towers and also other bleaching plant equipment, which results in high capital investment costs.
Concerning reductive bleaching agents and then primarily dithionite—normally sodium dithionite—besides the use of bleaching tower the bleaching agent can be added directly to the pulp suspension, for instance in a storage tower, therewith obviating the use of bleaching tower and other bleaching equipment. This latter alternative results in a lowering of the capital investment costs. Such known dithionite bleaching is normally effected in a temperature range of 40-60° C. In order to enhance the bleaching response, i.e. the bleaching efficiency, it has been suggested that the dithionite is charged directly to a refiner (see U.S. Pat. No. 5,129,987 owned by Joachimides et al, and a lecture entitled “Reductive Bleaching in Refiners”, Tappi Pulping Conference 1998, pp. 509-515). This method of procedure leads to an increased bleaching efficiency in comparison with a typical dithionite bleaching process, but manifested also drawbacks in the form of scaling within the refiner and a tendency towards corrosion damage.
In order to be successful when bleaching pulp with, for instance, dithionite, it is necessary to check and control the pH-value of the pulp suspension, the access of air to the pulp suspension, which should be restricted to the greatest possible extent, and the presence of hazardous and undesired metals in the pulp suspension, for instance transition metals.
Transition metals, particularly iron and manganese, are detrimental to the bleaching of mechanical pulp with, e.g., as well hydrogen peroxide as dithionite. The presence of manganese ions in significant quantities is particularly serious when bleaching pulp with hydrogen peroxide, whereas it is the iron ions that are particularly harmful when bleaching pulp with dithionite. These transition metals are normally removed from or neutralized in the pulp and the pulp suspension, by complex-binding the transition metals with a complexing agent, for instance in the form of ethylenediaminetetraacetic acid (EDTA) and/or diethylene triamine pentaacetic acid (DTPA). It has also been suggested that a reducing chemical, such as sodium hydrosulfite or sodium sulphite for instance, is added to the pulp suspension in addition to a complexing agent. Successes have also been achieved by treating wood chips solely with a complexing agent and with both of the aforesaid chemicals.